Khalid Sheikh Mohammed, the alleged September 11 mastermind, is seen shortly after his capture during a raid in Pakistan in 2003. Source: AP

THE alleged mastermind of the September 11 terrorist attacks has written a rambling, deeply religious manifesto that suggests Muslims should not use violence to spread Islam - a sharp departure from his earlier boasts of waging violent jihad against the US and other non-Muslim nations.

The unclassified comments by Khalid Sheikh Mohammed, who will soon turn 50 and who has spent the last decade in US custody, come a year and a half into the pre-trial phase of his military commission trial.

The writings suggest that he either believes he can convert his US audience or, as he often has done, is playing a mind game to attract publicity, feign mental illness or spare himself the death penalty if convicted in the 2001 attacks, the worst terrorist strikes in American history.

The 36-page document, titled "The Road to Real Happiness,'' includes not only references to the Koran, but quotes by Richard Nixon, George W. Bush and Pope Benedict XVI.

Mohammed rails against same-sex marriage, abortion and the moral decay of the West. He mocks US soldiers for playing with their PlayStations in Iraq and Afghanistan while Muslim families struggled for food and shelter.

"But at the end the American soldiers go back home and commit suicide but the poor man still with his dry bread and black tea lives with his poor wife in their humble muddy house but with happy hearts and souls,'' he wrote.

The document is described as the first of three parts, with subsequent writings to defend the September 11 attacks and dispute the validity of the US-led invasions of Iraq and Afghanistan, which he suggests were chiefly meant to benefit military contractors such as Halliburton Co.

The Koran, Mohammed wrote, "forbids us to use force as a means of converting'' others, and "truth and reality never comes by muscles and force but by using the mind and wisdom.''

Those statements clash with his earlier braggadocio in saying he plotted the September 11 attacks and personally beheaded Wall Street Journal reporter Daniel Pearl, and in calling for young Muslims around the world to embrace violence.

Mohammed personally addresses the military judge, court officials, prosecutors and defence lawyers involved in his case, inviting them to accept Islam and saying that his time in the US military prison at Guantanamo Bay, Cuba, has not been wasted.

He is "neither sad nor distressed,'' he wrote, "because I have been with the Only One True God.'' He added, "I am very happy in my cell because my spirit is free even while my body is being held captive.''

Pentagon officials said Mohammed wrote the document in October and asked that copies be hand-delivered to the court officials. He knew some of their names from ID tags they wore in court and asked his defence team to provide the others.

Defence lawyers took the document to the judge, and US intelligence and security officials at Guantanamo and elsewhere combed through it for references to classified material.

After the review, copies were shared with the court personnel. By Tuesday, the Huffington Post and Britain's Channel 4 News had obtained copies and made them public.

Mohammed grew up in Kuwait and was closely aligned with al-Qaeda leader Osama bin Laden. He is believed to have presented the plan for attacks using aeroplanes to bin Laden.

Mohammed was captured in 2003. After three years of custody at so-called CIA black sites, where he was repeatedly waterboarded, he was transferred to Guantanamo in 2006.

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